1. Field of the Invention
The present invention relates to a process and an apparatus for building tires for vehicle wheels.
2. Description of the Related Art
A tire for vehicle wheels generally comprises a carcass structure including at least one carcass ply having respectively opposite end flaps in engagement with respective annular anchoring structures, integrated into the regions usually identified as “beads”.
Associated with the carcass structure is a belt structure comprising one or more belt layers, located in radially superposed relationship with each other and with the carcass ply and provided with textile or metallic reinforcing cords having a crossed orientation and/or being substantially parallel to the circumferential extension direction of the tire.
A tread band is applied at a radially external position to the belt structure, which tread band is made of elastomeric material like other semifinished products constituting the tire.
Respective sidewalls of elastomeric material are further applied at an axially external position to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads.
After building of the green tire by assembly of respective semifinished products, a vulcanization and molding treatment is generally carried out, which aims at determining the structural stabilization of the tire by cross-linking of the elastomeric compounds and also at impressing the tread band, wound up around the carcass before vulcanization, with a desired tread pattern and the region close to the tire sidewalls with possible distinctive graphic marks.
Within the present description and in the following claims, by “elastomeric material” it is intended a composition including at least one elastomeric polymer and at least one reinforcing filler. Preferably, this composition further comprises additives such as cross-linking agents and/or plasticizers. Due to the presence of the cross-linking agents, this material can be cross-linked by heating, so as to form the final article of manufacture.
Document WO 2009/040534 in the name of the same Applicant, discloses a process for manufacturing tires for vehicle wheels, which process comprises the steps of: building the carcass structure of a green tire on a first forming drum in at least one carcass-building line, building a crown structure on a second forming drum in at least one crown-building line, shaping the carcass structure into a toroidal conformation, while it is being assembled to the crown structure in at least one assembly and shaping station. The assembly and shaping station is synchronized with the carcass-building line and the crown-building line. Each carcass structure is associated with the respective first forming drum on which it is built up to the end of the assembly and shaping step. The above mentioned stations are operatively associated with units adapted to supply elementary semi-finished products, such as continuous elongated elements of elastomeric material, strips of elastomeric material containing two or more textile or metallic cords, individual textile or metallic of coated with elastomeric material. Once the careen tire has been built, it is cured and molded in at least one vulcanization and molding line separated from the building line. In the above described work stations, said elementary semi-finished products can be wound up into coils disposed in side by side relationship and/or at least partly superposed, such as in the case of the continuous elongated elements for example, contributing to formation of the liner, under-liner, under-layers, fillers present in the regions of the beads, sidewalls, tread band.
In this regard, document EP 1 375 118 discloses a green tire, or a tire component, that is manufactured by winding into coils and rolling a rubber strip on a building drum. To this aim, a building device comprises a rigid support or building drum, on which the rubber strip is wound into coils and rolled, an extruder adapted to extrude the rubber strip, a winder for winding the rubber strip on the rigid support, and a flattening roller, adapted to flatten the projection of a step portion produced by a first part of the rubber strip and a second part of the same rubber strip overlapping the first one. Flattening prevents the residual air from remaining entrapped during vulcanization, in the regions close to the step portions between the outer surface of the tire component and the vulcanization mold and avoids the consequent presence of hollows and slits in the finished tire. The surface of the flattening roller is provided with a knurl. The step of spirally winding and rolling the rubber strip and the flattening step can be carried out simultaneously.
Document EP 1 754 592 discloses a method of building a tire, said method allowing elimination of the residual air between the step portions of an overlapped portion of a tire component and the vulcanization mold, so as to avoid formation of hollows, slits, etc. causing a reduction in the tire lifetime. The method comprises the steps of flattening a step formed at an overlapped portion of a tire component and optimizing the cross-section shape of said overlapped portion. The apparatus used for putting this method into practice comprises a building platform, an extruder for extrusion of a rubber strip and a pressure roller. A component of the tire is formed by spirally winding the rubber ribbon-like strip, extruded through the extruder, on the building platform and forming an overlapped portion of this rubber strip. A step of this overlapped portion that is exposed on the outer surface of the tire component is flattened under pressure through the pressure roller that is such disposed as to face the building platform by a specific angle relative to the extension direction of the rubber strip and is heated to the plasticization temperature in order to make the outer tire surface smooth.